12-23% is pretty easy, and includes some very prominent present-day cities. Detroit lost about 2/3 of its population between 1950 and today. Pittsburgh lost about 60%. St. Louis lost 65%. Buffalo lost 53%, Cincinnati and Baltimore around 40%.

Ghost towns are rarer, and it doesn't usually happen to major cities that have national recognition. There's a good list [1] on Wikipedia. tl;dr: they number in the hundreds, easily, but are largely small towns that you wouldn't have heard of unless you lived locally.

[1] https://en.wikipedia.org/wiki/Lists_of_ghost_towns_in_the_Un...